1. Field of the Invention
This invention relates to an image forming apparatus using an electrophotographic method and, more particularly, to an image forming apparatus such as a photocopier, printer, and facsimile machine.
2. Description of Related Art
Color image forming apparatuses generally form color images on a recording medium in a sheet shape or the like by transferring, in an overlapping manner, developers in four colors in total of yellow, magenta, cyan, and black.
The color image forming apparatuses are generally categorized into a single drum type having a single photosensitive drum and a multiple drum type (hereinafter referred to as “tandem type”) having a plurality of photosensitive drums. The single drum type needs to repeat an image forming process four times for images on one page, but the tandem type can form the four color developers step by step in an overlapping manner, so that the tandem type image forming apparatus is becoming major among the color image forming apparatuses targeting high speed operation.
The conventional tandem type color image forming apparatus, however, raises problems such as a tendency to increase maintenance costs as well as to render the apparatuses larger from the design to arrange the plural photosensitive drums in parallel.
To avoid the above problems, it is important to render optimized the replacement cycle of process cartridges including a photosensitive drum as a consumptive part, and image forming parts (namely, processing means) such as a charger, a developing unit, and a cleaner which are arranged around the photosensitive drum and to render the process cartridge itself compact to reduce the distance between the drums placed adjacently.
As a structure of the process cartridge, there are a so-called united body type cartridge in which developer containers for containing developers are incorporated as a united body, and a so-called supplement type cartridge in which the developer containers are separately formed. The united body type cartridge enjoys improved maintenance workability because all consumptive parts can be replaced at one time per color. The united body type cartridge requires replacing the cartridge according to the remaining amount of the developing agents even where the image forming parts do not yet reach the duration, thereby possibly increasing the maintenance costs, or requires increasing the amounts of the developing agents in matching the duration of the image forming parts, thereby possibly rendering larger the cartridges and the apparatuses.
With the tandem type color image forming apparatuses aiming at reduction of the maintenance costs and the apparatus sizes, it is desirable to use a supplement type cartridge structure which allows proper replacements of the developer containers according to the remaining developing agent amount and of the process cartridge according to the part's duration and in which the developer containers requiring an adequate volume are freely placed at locations apart from the process cartridge.
With the supplement type cartridge, however, supplying routes for developing agents extending from the developer containers to the process cartridge have to be newly arranged, whereas the developer containers are more freely placed so as to reduce the distances between the drums, so that the apparatus may be formed in a larger size with higher costs. Otherwise, where the developer container is placed adjacently to the process cartridge, the developing container and the process cartridge are necessarily detached in different directions to avoid interferences between the developing container and the process cartridge at attaching or detaching times, thereby possibly rendering worse the maintenance manipulation property.